Hello to all,
It is time to get rid of my 5 ton, seer 8 heat pump and replace it.
The house and the heat pump is almost 14 years old.
I live in the Denton Texas (DFW) area so it gets HOT in summer (heat index around 100 -110 from mid July to mid Sept). 
I have a two story home with standard 8 foot ceiling, somewhere around 3500 sq ft. Around 1500 down and 2000 up. (All of the full bathrooms are upstairs (humidity concerns)).

Winter although short can get as low as 18 degrees (overnight lows). Typical nights in Dec-Jan-Feb are in the high twenties to mid thirties so not too bad.
About 4 months of the year is mild and we only heat or cool occasionally.
The mild months we can have electric bills as low as $60.00.
When we get our cold snap in Jan we have to have the heat pump run continuously. It will run day and night for like 4 days and increases the monthly bill up to $330 for Jan.
Some summer bills can get around $300.
I had the current unit checked out a few years ago and it had plenty of refrigerant and I was told it was in good order but was not an efficient unit.

The home was built almost 14 years ago and we have always had cooling and heating challenges.
In the summer we have to close most of the downstairs vents (keeping the upstairs ones wide open) to keep the upstairs cool in summer.
We do the reverse in winter. Close the upstairs vents (leaving them slightly cracked) and open the downstairs fully to keep it warm.
I also added a small window unit upstairs (8000 btu, seer 10.8) and it has reduced my bill by $100 a month and made the upstairs so much more comfortable.
It paid for itself in and then some in one summer. I also added heat reflecting window tint that has also reduced temperatures in east and west facing rooms. Big help.

I have a plan that I am sure will need some scope changes.

Step 1. upgrade from a single 5 ton unit to two 3 ton units.
I have a fake wall next to my air handler that was put there for an extra air handler so I have the inside room.
I am thinking two 3 tons for the following reasons.
The current 5 ton cannot keep up in winter. It runs day and night and we freeze eventhough we are bundled up.
However it does a good job the rest of the year (providing we have half of the vents in the house shut).
I currently have 700 sq ft per ton and it does not do the job.  
This is a 3500 sq ft house with standard 8 foot ceilings. If I go with 600 sq per ton I will need (3500 / 600 = 5.8) or 6 tons.
I have been told that I may have to go with 500 sq ft per ton. Would 500 be overkill?

I have to add that the downstairs stays cool in summer due to the cold air sinking and the upstairs stays warmer due to the heat rising.
So I think I definitely need to stay with 3 ton units. Extra cooling for upstairs in summer (mostly hot in Texas).
3 ton down, for extra heating in winter and the excess will rise anyway.
Plus I plan to vent the returns so that the upstairs pulls from downstairs and downstairs pulls its air from upstairs. and of course I will split out the downstairs from the upstairs into two separate manifolds.

My question Any major or minor problems with step 1? any holes? Did I overlook something?


Step 2.

Do I go with a heat pump for upstairs and a heat pump with natural gas (dual fuel) for the downstairs?
The natural gas line runs down the middle of my attic and it is 1'' diameter. It goes to a fireplace with a log lighter that we almost never use. Or do I go with a GSHP?

I spoke to one guy who wanted $60k just to drill the wells? He wanted to punch five or six 200' holes in the backyard. $60k, no way.  Not in my budget.  

I know how to trench, so I can trench around the house and put in 1'' coil tubing. I am figuring on trenching 430 linear feet to a depth of 6 feet and about a foot wide.
The 1'' spiral tubing should be around1900 feet in length all curled up.
Each curl will be spaced a few inches apart (or does it have to be spaced)?
There is roughly .785375 sq inch per 1 inch tubing (1" I.D.) or is that cubic inches?
100 feet or 1200 inches of one inch (I.D.) tubing has an area (I hope that I am saying this right) of 943 cubic inches.
I am told that a gallon of water has roughly 231 cubic inches.
So 1200 inches of 1'' tubing should hold around 4 gallons of water. Pi * (1/2)^2 = .785375

If I have 1900 linear feet of tubing or 22,800 inches it should be able to hold 76 gallons of fluid.
if my calculations are off (and they very well could be, I was not a math major), wrong, incorrect, way out in left field, not even close please let me know what they should be?

thanks,

But if they are correct, would this be enough fluid to run two 3 ton units?


Step 3.
 
Closed Loop.
Once the length of tubing is verified Is it better to be in a continuous loop of 1900 linear feet?
or would I be better in splitting the 1900 into 3 separate loops?

The 1900 continuous loop seems like it would really have a better chance of normalizing temperatures but then I am having to deal with pressures and pump sizes.
If I use 3 loops will I need 3 pumps? That's 3 electric motors. Will that drive up costs?

Is it better to use 3 loops because it will reduce pressure?

I feel that most of the year I will either be using the upstairs or the downstairs. ONLY in extreme temperatures will I be using both the upstairs and downstairs but of course I want to design the system to run as if both were running.

Can both GSHP use the same common loop?
It would seem that they could as long as the flow rate is there?
Are there different stage pumps that could increase flow as needed?
are there probes that monitor the temp of the fluid going back out into the system buried in the yard as opposed to the fluid that is returning?
Is it possible to use an additional tank so that the amount of fluid in the system is greater?
I would set it up like a power steering pump on a car so that the normalized fluid comes in the top and goes out the bottom into the GSHP and then back out into the yard (but only if needed)?

I think that is enough questions for now.

Thanks

Does the attic need more insulation before doing a manual J calculation?  I did not see anything in your posting about any steps that may be possible to reduce the HVAC load.  I am no expert on HVAC but I think it would be less costly to reduce the load instead of buying bigger equipment.  I know in some cases adding insulation is not possible.  Do you plan to use the existing ducts?  If so, have they been checked for leaks?

Posted By help-me-conserve on 22 Aug 2010 07:18 PM

I feel that most of the year I will either be using the upstairs or the downstairs. ONLY in extreme temperatures will I be using both the upstairs and downstairs but of course I want to design the system to run as if both were running.

Can both GSHP use the same common loop?
It would seem that they could as long as the flow rate is there?

Thanks for your replies.

I do plan to add some more insulation in the attic.    I have between 8 to 11 inches of the blown-in pink stuff and all of the baffels look clear so I have airflow.

I have added whirly bird vents and started a radiant barrier system using aluminum foil.

It looks like I need to get a Manual J done as my next step.

So a continuous 1900 foot loop using 1'' line is not a good idea?  So I should split that in two if I am using two 3 ton units?

Posted By help-me-conserve on 23 Aug 2010 09:23 AM
So a continuous 1900 foot loop using 1'' line is not a good idea?

Posted By Texas Cooler on 24 Aug 2010 05:01 PM
I'm just north of Denton with with lots of TX HVAC experience. Your situation with hot/cool is common without zoning and RNC contractor ductwork. Most of the ductwork I see is the R2 flex that is not installed correctly and improperly designed and poorly sealed. Unless you have a lot of land and deep soil to rock, vertical is the best way for geo in our area. Also, package unit retro-fits are difficult when there is no basement to bring the heat exchange lines in below the surface although anything is possible with enough $$$. It is possible to have a geo split outdoors and with only the air handler upstairs. If your bills are only $300/month during the worst times of the year, the ROI for two geo systems would be hard to justify. The first step should be a heat load analysis to determine exactly HOW MUCH heat/air you need. The analysis should also detail how much airflow is needed for heating/cooling in each room. While doing this the technician can also measure the static pressure of the system which can help determine how well it is performing with your current air handler. When this is all computed, you may see that the ductwork installed will not perform regardless of what kind of system you have and may need to be replaced if you expect to receive the efficiency claimed. Lastly, a geo heat exchanger loop system needs to be correctly installed and designed for the installed equipment and site. For the 2 ton geo system I am installing in my workshop office using horizontal trenching, I have to install 1150' for EACH ton buried 7'deep. Additionally, I'm installing a drip sprinkler below the surface just in case we go thru another drought. I'm going horizontal vs. vertical because I have a backhoe, dozer and truck , the available land and it will be cheaper for me to install. I also want to prove to myself that a properly designed and installed horizontal will work every bit as well as the vertical bores.

Posted By help-me-conserve on 24 Aug 2010 06:11 PM
I have heard where somone used copper tubing because copper is 19 times better for therm transfer rate. After doing a pressure test they filled the trench with grout to keep it from being crushed and then refilled the trench.
has anyone else heard of this?

Posted By help-me-conserve on 24 Aug 2010 06:11 PM
I have heard where somone used copper tubing because copper is 19 times better ...

Posted By engineer on 24 Aug 2010 11:43 PM

Posted By help-me-conserve on 24 Aug 2010 06:11 PM
I have heard where somone used copper tubing because copper is 19 times better for therm transfer rate. After doing a pressure test they filled the trench with grout to keep it from being crushed and then refilled the trench.
has anyone else heard of this?

Selecting Copper for its 19x heat transfer rate is a bit like specifying that you will only buy a car capable of 250 mph and then using it on US public highways.

In other words, the copper can superbly conduct all the heat you extract or reject into whatever grout or backfill you choose, and run smack into a heat transfer 'speed limit' of 55 mph imposed by the much lower conductivity of the immediately surrounding material.

Posted By engineer on 24 Aug 2010 11:39 PM
Spot on, especially for post number one.

Well done, full marks, welcome aboard.

Keep us advised as your project unfolds.

Posted By help-me-conserve on 23 Aug 2010 09:23 AM

Thanks for your replies.

I do plan to add some more insulation in the attic.    I have between 8 to 11 inches of the blown-in pink stuff and all of the baffels look clear so I have airflow.

I have added whirly bird vents and started a radiant barrier system using aluminum foil.

It looks like I need to get a Manual J done as my next step.

So a continuous 1900 foot loop using 1'' line is not a good idea?  So I should split that in two if I am using two 3 ton units?